Asthma and wheezing illness are important causes of impaired quality of life, health care utilisation, consumption of prescribed drugs and hospital admission in children (Anderson et al., 2007). While some of the environmental causes of elevated childhood asthma morbidity are relatively well established, the role played by domestic pets in the development of asthmatic symptoms remains unclear. We propose here to carry out a detailed analysis of a large scale longitudinal study in which detailed prospective information about cat, dog and other pet exposures have been collected at a number of key time points and combined with objective measures of asthma occurrence and allergic sensitization. We will do this in the context of careful statistical adjustments for potential mediators and confounds. The research study we propose will represent the largest and most comprehensive investigation to date of the associations between household pet ownership and the development of childhood asthma. Over half of UK households own a pet of some type, while in the US, pet ownership is even more common. The potentially beneficial impact of companion animal ownership on the physical, psychological and social well-being of children has been widely discussed in recent years. But decisions concerning how to weigh this against possible negative effects, such as an increased incidence of asthma and wheeze in pet owning children, ultimately depend on reliable empirical data that can only be supplied by large scale birth cohort studies such as the one proposed here. A recent review of the role played by cats and dogs in the development of asthma and allergy concluded that, to date, no definitive conclusions can be drawn concerning their deleterious or protective effects (Chen et al, 2010). We propose to make use of the rich longitudinal data collected as part of the Avon Longitudinal Study of Parents and Children (ALSPAC), a British-based, ongoing, cohort study of more than 14,000 children born in the early 1990s. This will provide us with detailed information concerning the numbers and types of pets kept in participant children's households from gestation through repeated (approx. yearly) sampling points across childhood until age 8 years. It will allow us to investigate any associations between these and a number of objectively measured outcome measures (inc., atopic sensitization to animal and other allergens, and a variety of lung function scores), and parentally reported outcome variables (maternally reported wheeze, doctor diagnosed asthma) whilst also allowing us to control for potential confounds and moderators.